1. Field of the Invention
The present invention relates to a light receiving amplification element to be used for an optical pickup apparatus using a semiconductor two-wavelength laser element which outputs light with two different wavelengths from one chip.
2. Description of the Related Art
Optical disks such as CD, DVD and the like are widely utilized as media on which a voice, an image, document data or the like is recorded and development is performed for various types of optical disk apparatuses which performs reproducing or recording the optical disk. An optical pickup apparatus is a main component for performing an input/output of a signal to/from the optical disk in a head portion of the optical disk apparatus.
Since a DVD reproduction apparatus, which prevails rapidly in recent years, requires capability of reproducing an existing CD, the optical pickup apparatus requires two semiconductor laser elements having 650 nm wavelength for DVD and 780 nm wavelength for CD, respectively.
FIG. 7 is a side view showing an optical pickup apparatus for performing reproduction of DVD and CD.
Referring to FIG. 7, an optical pickup apparatus 101 is provided with two semiconductor laser elements 107 and 108 each of which acts as a light source. The semiconductor laser element 107 outputs (emits) a light having 650 nm wavelength for DVD and the semiconductor laser element 108 outputs (emits) a light having 780 nm wavelength for CD. A prism 106 and an optical lens 103 comprising a condensing lens 104 for DVD and a condensing lens 105 for CD are arranged on a light path from the semiconductor laser elements 107 and 108 to an optical disk 102.
In addition, the optical pickup apparatus 101 is provided with a light receiving amplification element 109 having a light receiving portion (a photo diode) for receiving laser light which is reflected by the optical disk 102 and is reflected by the prism 106 via the optical lens 103. The optical lens 103 is provided with a function of switching between semiconductor laser elements 107 and 108 so as to correspond to a selected wavelength and simultaneously switching between condensing lenses 104 and 105.
When DVD (wavelength: 650 nm) is selected, the optical disk 102 is irradiated with emitting light from the semiconductor laser element 107 via the condensing lens 104 and light reflected by the optical disk 102 is reflected by the prism 106 via the condensing lens 104 and is inputted in the light receiving amplification element 109 and is photoelectrically converted by the light receiving amplification element 109 and an electric signal is outputted. When CD (wavelength: 780 nm) is selected, the optical disk 102 is similarly irradiated with emitting light from the semiconductor laser element 108 via the condensing lens 105 and light reflected by the optical disk 102 is reflected by the prism 106 via the condensing lens 105 and is inputted in the light receiving amplification element 109.
Since a light path can be adjusted by the two condensing lens 104 and 105 with respect to two light sources (the semiconductor laser elements 107 and 108), the light receiving portion (the photo diode) on the light receiving amplification element 109 has a shape shown in FIG. 8 and a light receiving portion 201 comes into receiving a laser light for 650 nm wavelength for DVD as well as 780 nm wavelength for CD. Here, focus adjustment and reproduction of a data signal are performed in a main light receiving portion 202 which is arranged in the center of the light receiving portion 201 and is divided into 4 portions. In addition, tracking adjustment is performed in sub-light receiving portions 203 arranged on the right and left sides of the main light receiving portion 202.
Next, FIG. 9 shows an equivalent circuit block diagram of the light receiving amplification element according to the prior art.
In FIG. 9, the whole of the block diagram is denoted as a light receiving amplification element 301, and a light receiving portion 302 (the photo diode) corresponds to each of A through D of the main light receiving portion 202 and E through H of the sub-light receiving portion 203. A laser light signal is converted to an electric signal Isc in the light receiving portion 302 and the signal Isc is amplified by current-voltage conversion by a resistance R11 of a pre-amplifier 11 and voltage of the signal is more amplified by a post-amplifier A12, resistances R12, R13 and R14 and the signal is outputted to an output terminal 303.
In the optical pickup apparatus in FIG. 7 described in the prior art, two semiconductor laser elements 107 and 108 and two condensing lenses 104 and 105 respectively corresponding to two different wavelengths (650 nm, 780 nm) are required and it is required that reflecting (signal) light is guided from the optical disk to the light receiving amplification element by adjusting an optical system such as a position of the semiconductor laser element, a position of the condensing lens or the like for each of the two wavelengths. Consequently, since optical design including the semiconductor laser element is complicated and there is necessity of a mechanism available for switching the optical system between the two wavelengths, the optical design becomes more complicated and the number of parts is increased and there is progress related cost is also increased.
Consequently, as a semiconductor laser element, there has been advanced development and commercialization of a semiconductor two-wavelength laser element which outputs (emits) light with two different wavelengths from one chip and there has been adopted a single semiconductor two-wavelength laser element for the optical pickup apparatus instead of using two semiconductor laser elements.
Advantage of the semiconductor two-wavelength laser element is capability for making a distance between two-wavelength light emitting points closed in order of μm and with extremely high accuracy of positions of the two-wavelengths light emitting points because the two wavelengths light emitting points are formed on a same substrate by using a technique of semiconductor photograph. Therefore, even when the optical disk is irradiated with laser light via one condensing lens and reflecting (signal) light from the optical disk is guided to the light receiving amplification element, a distance between receiving light points for two wavelengths is fixed with high accuracy.
As mentioned above, on the assumption that the number of the semiconductor laser elements and the number of the condensing lenses are defined as one respectively, there is necessity for the light receiving portion (i.e. the photo diode) corresponding to two wavelengths on the light receiving amplification element. In addition, there is necessity for a circuit amplifying an electric signal which is photoelectrically converted from the light receiving portion corresponding to each of the two wavelengths.